Without limiting the scope of the invention, its background is described in connection with photonic crystal waveguides (PCW). Introducing line defects into a photonic crystal lattice permits an electromagnetic wave having a frequency within the bandgap of the structure to be guided through the photonic crystal. The line defects resemble waveguides, and may be formed by either adding or removing dielectric material to a certain row or column along one of the directions of the photonic crystal lattice. Thus, PCWs can be used as an optical “wire” to guide an optical signal between different points, or devices, within an optical integrated circuit.
Slotted photonic crystal waveguides offer a unique platform that merges the best properties of slot waveguides and photonic crystal waveguides (PCW): strong optical confinement in slot waveguides [1] and slow light-enhanced light-matter interaction in photonic crystal waveguides. [2] In a W1 PCW, the optical mode profile spreads deeper into the photonic lattice with reduced group velocity. [3] This lateral spread reduces optical confinement and increases propagation loss for slow light modes, which can weaken some of the benefits derived from the slow light effect. By contrast, in slotted PCW, optical confinement does not decrease with increased group index, as a result of the high index contrast in silicon platform. In a high index contrast interface, a transverse electric (TE) guided mode is required to have much higher intensity in the low index region as stated by Maxwell's equations. Consequently, when approaching the edge of the photonic bandgap, the percentage of energy concentration in the low index slot will increase rather than decrease. [4] The increasing optical confinement with slower group velocity is a very advantageous property for compact optical communication [4-6] and sensor devices [7,8] that require strong light-matter interaction and high optical confinement simultaneously. Despite the benefits derived from strong confinement and slow light enhancement, optical coupling between a strip waveguide and a slotted PCW is more challenging than conventional PCW due to the exotic mode profile and slow group velocity in the slotted PCW. Without a properly designed interface between a strip waveguide and a slotted PCW, strongly confined guided mode profile with minimal overlap and large group index mismatch result in negligible coupling and a transmission spectrum that is nearly indistinguishable from noise [9]. Efforts to improve the coupling efficiency between a strip waveguide and a slotted PCW include using a multi-mode interference (MMI) coupler [9], changing the termination of the slot with respect to the coupler interface [10,11], and resonant coupling [15]. Using an MMI coupler to improve mode-matching yields less than 5 dB insertion at the peak wavelength. However, efficient coupling is achieved only within a narrow bandwidth. Changing the slot termination profile achieves good coupling with significantly better bandwidth [10], but the overall transmission is low, and Fabry-Perot fringes manifest due to significant reflections. Resonant coupler approach shows better coupling efficiency. However, the transmission dip below −10 dB in slow light region weakens the performance of slow light devices. By contrast, the theoretical study in [16] suggests that good coupling is achievable with good mode profile and group index matching.
As a result, photonic crystal waveguides are a very promising platform for ultra-compact optical devices with relatively low power consumption. However, optical loss in photonic crystal waveguides is the most significant stumbling block, and coupling loss in photonic crystal devices is the dominant optical loss.
For example, U.S. Pat. No. 7,231,122 issued to Weisberg et al. (2007) features an apparatus that includes a photonic crystal fiber configured to guide a mode of electromagnetic radiation at a wavelength, λ, along a waveguide axis. The fiber includes a core extending along the waveguide axis, and a confinement region extending along the waveguide axis and surrounding the core. The confinement region includes alternating layers of a first and a second dielectric material having thicknesses d1 and d2 and different refractive indices n1 and n2, respectively. The thickness of at least one of the alternating layers of the first material differs from thickness d1QW or at least one of the alternating layers of the second material differs from thickness d2QW, where d1QW and d2QW correspond to a quarter-wave condition for the two dielectric materials given by d1QW=λ/(4√{square root over (n12−1)}) and d2QW=λ/(4√{square root over (n22−1)}), respectively. The photonic crystal fiber has an attenuation for the guided mode at the wavelength λ that is reduced by a factor of about two or more relative to an attenuation for a reference fiber that is identical to the photonic crystal fiber except that the reference fiber has alternating layer thicknesses corresponding to the quarter-wave condition.
U.S. Pat. No. 7,535,944 issued to Guilfoyle (2009) discloses a photonic crystal/waveguide coupler for VCSELS and photodetectors. The optical coupler of the Guilfoyle invention comprises first and second mirrors. The first mirror is positioned with respect to the second mirror so that a resonant cavity is defined between them. A waveguide structure is positioned in the resonant cavity and includes a photonic crystal coupler. A thickness of the resonant cavity is selected so that a phase matching condition is satisfied for resonance in the resonant cavity. At least one of the first and second mirrors may be formed from a structure in an optoelectronic device. Alternatively, at least one of the first and second mirrors is formed from a semiconductor layer. At least one of the first and second mirrors may be formed as a semiconductor distributed bragg reflector, or as a dielectric distributed bragg reflector. At least one of the first and second mirrors may be a mirror in a vertical cavity surface emitting laser (VCSEL) structure. The photonic crystal coupler structure may be shaped so that first order modes of light incident upon the photonic crystal coupler structure are coupled into the waveguide while zero-order modes are reflected out into the resonant cavity and reflected by the mirror.